The present invention relates to a adhesive agent for a substrate for electroless plating of printed circuit board, an adhesive film which is made of a base film coated with the adhesive agent, and a printed circuit board using the adhesive agent.
A printed circuit board, in which circuit is formed by electroless plating on surface of an insulating substrate, has an adhesive layer on the substrate which is formed for adhesion of films of electroless plating.
As for the adhesive agent of the adhesive layer, agents which are composed mainly of epoxy resin, synthetic rubber, and phenolic resin (JP-B-45-9843 (1970), JP-A-58-57776 (1983), JP-A-59-62683 (1984), JP-A 62-248291 (1987), JP-A-63-277772 (1988)), and of synthetic rubber and phenolic resin (JP-B-60-5079 (1985), JP-A-63-213676 (1988), JP-A-63-213677 (1988)) are disclosed.
As for the adhesive agents described above, the adhesive agents comprising imidazole group compounds, amine group compounds, acid anhydride compounds, and novolak type phenolic resin etc. as a hardener for the epoxy resin, and further, resol type phenolic resin and sulfur compounds etc. as a vulcanizing agent, and metal oxides such as zinc oxide and magnesium oxide etc. as a vulcanization accelerator are used.
The adhesive layer described above is etched with a mixture of chromic acid and sulfuric acid etc. for improving of peeling strength and resistance against soldering heat of the film of electroless plating by making a surface of the layer rough and adding an anchor effect to the surface. Further, to raise the anchor effect, some of the adhesive layer comprises inorganic fillers such as calcium carbonate, calcium silicate, silicon oxide, zirconium silicate, and titanium oxide.
The adhesive layer described above is formed by a method comprising the steps of adding solvent to the adhesive agent, applying the solution on the surface of the insulating substrate by any of dipping process, roll coating process, and curtain coating process etc., drying, and hardening by heating.
Conventionally, heating at 150.degree.-200.degree. C. for 30-120 minutes is necessary for hardening of the adhesive agent described above as disclosed in JP-A-62-248291 (1987).
But actually, even though the heating at 150.degree.-200.degree. C. for 30-120 minutes is able to harden epoxy resin, the heating is insufficient for reaction of synthetic rubber with its vulcanizing agents such as phenolic resin or sulfur compounds, and consequently, problems of low insulation resistance of the adhesive layer, swelling of the adhesive layer by stress with deposition of the plating film at the electroless plating, and low peeling strength and low resistance against soldering heat of the plating film are caused. Therefore, generally speaking, hardening by heating at lowest 160.degree. C. for at least 60 minutes is performed actually as disclosed in JP-A-62-248291 (1987).
Recently, thickness of a printed circuit board is becoming thinner year by year, and 0.8-1 mm in a case of paper phenolic laminate board and 0.5-1 mm in a case of glass epoxy laminate board is required for thickness of the insulating substrate. And, a fine circuit having at most 0.1 mm for width of a conductive circuit which is formed on the surface of the insulating substrate or for a space between the conductive circuits is required. By forming the adhesive layer on such a thin insulating substrate and hardening at lowest 160.degree. C. for at least 60 minutes, warp and distortion of the insulating substrate occurs, and a problem of difficulty in printing of resist ink and adhesion of dry film type resist, due to the warp and the distortion of the substrate, in a plating resist coating process which is a preprocess of the electroless plating, is caused.
In a case of using a paper phenolic laminate board as the substrate, the paper in the substrate is deteriorated by heat, and there is a problem of causing cracks between next through holes and between perforated small holes, and of cutting sections becoming worm-eating pattern.
On the other hand, although high insulation resistance is required for an interval between conductive circuits which are formed by electroless plating, a value of insulation resistance after hardening of an adhesive layer using a conventional adhesive agent is about 10.sup.11 -10.sup.13 .OMEGA., and the value is not sufficient for the insulation resistance of the adhesive layer upon which a fine circuit, having an interval between circuits of at most 0.1 mm, is mounted.
One of the objects of the present invention is to provide an adhesive agent for a substrate for electroless plating, which is hardened at relatively low temperature (about 150.degree. C.) in a short time (within 30 minutes) without giving warp and distortion to the substrate, and, further, yields an adhesive layer having high insulation resistance after the hardening, and a printed circuit board using the adhesive agent.
Further, another object of the present invention is to provide an adhesive film which is formed by coating of the adhesive agent on a base film and yields an adhesive layer for a substrate for electroless plating.